The 19” rack is a standard that precedes computers, so you can find a lot of non-computer gear that fits in them. Synthesisers and studio audio gear are one example, then there’s test equipment and such. The military undoubtedly has a lot of specialised gear that also fits 19” racks (the form factor may have originated there, perhaps during WW2). It wouldn’t be surprising if there was non-electrical gear that was designed for mounting in these universally available racks.